Laser trackers are used for industrial measurement, e.g. for coordinative position determination of points of a component such as a vehicle body for example in the context of an inspection or for continuous position monitoring of a moving machine part. Such laser trackers are designed for coordinative position determination of said target point and usually for continuous tracking of a retroreflective target point. In this case, a target point can be represented by a retroreflective unit (e.g. cube prism) which is targeted by an optical measurement beam generated by a beam source of the measuring device or of the measuring apparatus, in particular a laser beam. The laser beam is reflected back to the measuring apparatus in a parallel fashion, the reflected beam being detected by detection means of the apparatus. In this case, an emission direction and respectively a reception direction of the beam are ascertained, for example by means of sensors for angle measurement which are assigned to a deflection mirror or a targeting unit of the system. In addition, with the detection of the beam, a distance from the measuring apparatus to the target point is ascertained, e.g. by means of time-of-flight or phase difference measurement or by means of the Fizeau principle. The position coordinates of the target point are determined on the basis of the emission direction and respectively the reception direction and the distance.
For distance measurement, laser trackers in the prior art comprise at least one distance measuring device, wherein the latter can be designed e.g. as an interferometer (IFM). Since such distance measuring devices can measure only relative changes in distance, in order to determine an absolute distance value so-called absolute distance measuring devices (ADM) are installed in present-day laser trackers. A combination of an absolute distance measuring device and an interferometer for determining distance is known from WO 2007/079600 A1, for example.
In addition, in modern tracker systems, an offset of the received measurement beam from a zero position is ascertained on a fine targeting sensor. By means of this measurable offset, it is possible to determine a difference in position between the center of a retroreflector and the impingement point of the laser beam on the reflector and it is possible to correct or readjust the alignment of the laser beam depending on this deviation in such a way that the offset on the fine targeting sensor is reduced, in particular is “zero”, and the beam is thus aligned in the direction of the reflector center. As a result of the readjustment of the laser beam alignment, continuous target tracking of the target point can be carried out and the distance and position of the target point can be determined continuously relative to the measuring device. The readjustment can be realized in this case by means of a change in alignment of the deflection mirror provided for deflecting the laser beam, said deflection mirror being movable in a motorized manner, and/or by pivoting of the targeting unit comprising the beam-guiding laser optical unit.
The target point or the retroreflector can be fitted in this case on an auxiliary measuring instrument, e.g. a so-called contact sensing tool that is positioned with a contact point on a point of the object to be measured. The contact sensing tool comprises markings, e.g. light points, and a reflector, which represents the target point on the contact sensing tool and is targetable by the laser beam of the tracker, the positions of the markings and of the reflector relative to the contact point of the contact sensing tool being known precisely.
What is not possible with customary laser trackers in the prior art is the measurement of distances without the use of measuring aids comprising a retroreflector, i.e. measurement directly to a surface of an object to be measured. Primarily, with such laser trackers, can scanning measurement of surface points is possible, that is to say determination of very many point coordinates, which is carried out in a comparatively short time. For this purpose, however, losses in accuracy have to be accepted in comparison with the measurement of retroreflective targets.
US 2014/0226145 A1 discloses a laser tracker which can measure both a retroreflective target and a natural (that is to say non-retroreflective) surface. For this purpose, the laser tracker comprises a first absolute distance measuring device, which is designed as known for measurement with respect to a retroreflector. In addition, the laser tracker comprises a second ADM, which is designed for measurement with respect to an object surface. Although the respective ADMs transmit their measurement radiation through a single exit optical unit, they are in each case separate, independent units. The need to provide two completely independent, separate absolute distance measuring devices is disadvantageously complex in terms of production engineering and thus expensive.